The invention relates to a process for the preparation and final purification by distillation of 3,7,11,15-tetramethyl-1,2,3-trihydroxyhexadecane (phytantriol) of 
which is in demand as an addition to skin and hair care compositions.
Phytantriol is in demand in the cosmetics industry as humectant and as penetration enhancer for cosmetic active ingredients such as panthenol, vitamin A and vitamin E (cf., for example, DE 42 15 501, EP 686 386, EP 679 383, DE 42 34 744, JP 04069316, JP 61236737, DE 14 67 925 and DE 11 42 428).
Essentially two industrial processes for preparing this active ingredient are known.
1) DE 1 149 700 discloses a process for preparing trihydric alcohols of the general formula II 
in which tertiary alcohols of the general formula III 
such as isophytol (n=3) can be reacted in a manner known per se with an organic peracid, in particular with performic acid, followed by alkaline hydrolysis to prepare phytantriol (I).
The disadvantage of this process is that the resulting trihydric alcohols, which easily decompose at high temperatures, are heavily contaminated by byproducts which have approximately equally high boiling points, and therefore cannot be prepared in the purity required for cosmetic purposes even on fractional distillation under high vacuum (about 10xe2x88x921 to 10xe2x88x925 mbar) such as short-path distillation or molecular distillation. Apart from the fact that such separations under high vacuum entail both very high capital costs and high running costs, only insufficiently pure products can be obtained at justifiable expense from such mixtures with low relative volatilities. High purities are obtained only with extremely complicated apparatus and extremely low distillation yields.
2) Japanese Patent Publication JP 61236737 A2 discloses a process for preparing phytantriol in which the latter is obtained in somewhat greater purity. In this process, isophytol is reacted in the presence of vanadium or molybdenum compounds with tert-butyl hydroperoxide [(CH3)3 Cxe2x80x94OOH], and an opening of the epoxide rings in the resulting epoxy compounds is then carried out with acidic catalysts. The crude phytantriol obtained in this way is then purified by molelcular distillation under 0.22 mbar.
The disadvantages of this process are the use of heavy metals, which is undesirable in the cosmetics industry, the relatively elaborate process management and the need to purify by molecular distillation.
It is an object of the present invention to prepare phytantriol, which is in demand as active ingredient in the cosmetics industry, in very pure form on an industrial scale in a simple manner without using heavy metal compounds and without the need to purify under high vacuum.
We have found that this object is achieved by subjecting contaminated phytantriol, in particular the phytantriol which can be prepared in a relatively simple manner by the process of DE 1 149 700 but is contaminated with lower and/or higher boiling colored additional components to a very specific rectification under medium vacuum (i.e. with an overhead vacuum of from 0.1 to 2 mbar) to result in very pure product in a high distillation yield without costly molecular distillation.
Although DE 19524928 discloses a process for obtaining pure substances from mixtures of high boiling air- and/or temperature-sensitive substances requiring high separation efficiency by rectification under specific conditions under medium vacuum, it was nevertheless very surprising that even the high boiling phytantriol, which is extremely sensitive to thermal stresses, can be rectified under medium vacuum with distillation yields of up to 95% of theory to give high-purity phytantriol (purity greater than 99%). This is particularly true since it is evident from the UIC-News of Norbert Kukla UIC GmbH of Dec. 7, 1998 that even one of the significant manufacturers of phytantriol, despite intensive research into the ultrapurification of phytantriol, regards only short-path distillation, i.e. a very complex distillation under extremely high vacuum, as suitable, although the research, as demonstrated by the statements in said UIC-News, had the aim of meeting not only higher quality requirements but also the understandable requirement for increased productivity and reduction of costs.
The invention accordingly relates to a process for preparing high-purity phytantriol, which comprises rectifying phytantriol which is contaminated with lower and/or higher boiling colored byproducts under medium vacuum in columns containing metal packings with ordered structure, wherein
a) the liquid distribution is carried out by channel distributors with a minimum of 500 drip points per m2,
b) the channels of the liquid distributors are arranged at an angle of about 90xc2x0 to the cloth layers of the packing elements located directly below these distributors,
c) 2 or more packing elements which have a height of only 20 to 100 mm and whose cloth layers are each rotated by 90xc2x0 relative to one another are inserted underneath the liquid distributors,
d) air is absolutely excluded during operation, and
e) the rectification is carried out strictly adiabatically.